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<a name="Function-Names"></a>
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<hr>
<a name="Function-Names-as-Strings"></a>
<h3 class="section">6.50 Function Names as Strings</h3>
<a name="index-_005f_005ffunc_005f_005f-identifier"></a>
<a name="index-_005f_005fFUNCTION_005f_005f-identifier"></a>
<a name="index-_005f_005fPRETTY_005fFUNCTION_005f_005f-identifier"></a>

<p>GCC provides three magic constants that hold the name of the current
function as a string.  In C++11 and later modes, all three are treated
as constant expressions and can be used in <code>constexpr</code> constexts.
The first of these constants is <code>__func__</code>, which is part of
the C99 standard:
</p>
<p>The identifier <code>__func__</code> is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration
</p>
<div class="smallexample">
<pre class="smallexample">static const char __func__[] = &quot;function-name&quot;;
</pre></div>

<p>appeared, where function-name is the name of the lexically-enclosing
function.  This name is the unadorned name of the function.  As an
extension, at file (or, in C++, namespace scope), <code>__func__</code>
evaluates to the empty string.
</p>
<p><code>__FUNCTION__</code> is another name for <code>__func__</code>, provided for
backward compatibility with old versions of GCC.
</p>
<p>In C, <code>__PRETTY_FUNCTION__</code> is yet another name for
<code>__func__</code>, except that at file scope (or, in C++, namespace scope),
it evaluates to the string <code>&quot;top level&quot;</code>.  In addition, in C++,
<code>__PRETTY_FUNCTION__</code> contains the signature of the function as
well as its bare name.  For example, this program:
</p>
<div class="smallexample">
<pre class="smallexample">extern &quot;C&quot; int printf (const char *, ...);

class a {
 public:
  void sub (int i)
    {
      printf (&quot;__FUNCTION__ = %s\n&quot;, __FUNCTION__);
      printf (&quot;__PRETTY_FUNCTION__ = %s\n&quot;, __PRETTY_FUNCTION__);
    }
};

int
main (void)
{
  a ax;
  ax.sub (0);
  return 0;
}
</pre></div>

<p>gives this output:
</p>
<div class="smallexample">
<pre class="smallexample">__FUNCTION__ = sub
__PRETTY_FUNCTION__ = void a::sub(int)
</pre></div>

<p>These identifiers are variables, not preprocessor macros, and may not
be used to initialize <code>char</code> arrays or be concatenated with string
literals.
</p>
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